Process for preparing silica gel



United States Patent 0.

3,203,760 PROCESS FOR PREPARING SILICA GEL Milton E. Winyall,Gainesville, Fla, assignor to W. R. Grace & (10., a corporation ofConnecticut No Drawing. Filed Dec. 17, 1962, Ser. No. 244,940 Theportion of the term of the patent subsequent to Dec. 25, 1979, has beendisclaimed 2 Claims. (Cl. 23-182) and that the amount adsorbed dependson the nature and extent of the surface. For example, commercial silicagel contains an extensive lattice of void spaces which permitsadsorption of approximately 50% of its weight from saturated air. Fromthis, it has been estimated that one cubic inch of silica gel containspores having a surface of about 50,00 square feet. With this enormousinternal surface and infinite number of small diameter capillaries,silica gel attracts vapors, condenses them, and holds them as liquids.The same phenomenon applies to the adsorption of liquids per se.

The silica prepared according to this invention has the same chemicalcomposition as silica gel and differs physically from the highly porousgel. While the gel is characterized by a high surface area, i.e., of theorder of 300 to 1000 m. /g., the silica of this invention possesses asurface area below 250 m. /g., and generally between and 250 m. g.Consequently, this silica lacks the characteristic adsorptive propertiesof silica gel. The absence of appreciable adsorptivity renders the thesilica of this invention suitable for such uses as a filler, forexample, in asphalt or vinyl type tiles. In addition, the silica iscolorless, and for this reason it finds particular applicability as afiller in pastel vinyls which, while providing the costreducing bulk,does not affect the chromaticity of the final product.

In the parent case, a process for preparing finely divided colorlesssilica having a low surface area was described. The process comprisedforming an aqueous solution of a suitable silicate, preferably on alkalimetal silicate, spray drying the silicate solution, acidifying the driedsilicate particles with agitation and thereafter purifying the same. Theparent application described products having a surface area of about 10to about 100 m. /g., preferably about 10 to 50 mF/g.

This application covers the product recovered from the process of theparent case and the product recovered from variations of this process,which are new products having a surface area of up to about 250 m. g.

In carrying out the processes, a silicate solution is prepared, forexample, by dissolving a soluble silicate such as an alkali metalsilicate in water. Although any of the alkali metal silicates may beused, such as sodium silicate, potassium silicate, etc, sodium silicateis preferred because of its favorable economic position. For the purposeof simplicity, therefore, the invention will be described by referenceto the use of sodium silicate although it is to be understood that it isnot limited to the use of this material. The starting silicate solutionmay have an SiO :Na O weight ratio of from about 1:1 to 3.40:1.

3,203,760 Patented Aug. 31, 1965 ice The silicate solution is thensprayed as a fine mist into a drying chamber heated by suitable means,such as the combustion of propane. The mist is introduced into the topof the spray dryer, where it contacts a countercurrent flow of hot gasesin its downward flight, and settles at the bottom as solid, dry,spheroidal particles of substantially uniform size and shape. Control ofthe particle size may be effected by controlling the characteristics ofboth the nozzle or spraying device and the silicate solution. Forinstance, the pressure applied in introducing the solution through thespray nozzle, its viscosity, temperature and composition will influencethe particle size. Likewise, the type of nozzle or its specificconstruction also affects the particle size. The resulting particles aresmall and of suflicient hardness so that the rate of fall is of suchgentle nature that very little fracture occurs whenthey reach the bottomof the dryer. Examination of the collected spray dried particlesrevealed that the shape was substantially spheroidal with less than 1%-of the particles being irregularly shaped.

The dried particlesare then treated with a suitable weakly acidicsolution, such as 12% ammonium sulfate solution or a mineral acid suchas sulfuric, hydrochloric, nitric acid, etc. The acidic material isadded as a weak solution and in an amount sufficient to neutralize allthe soda in the dried silicate particles and is of sufiicient strengthto keep the pH of the treated particles on the acid side. Acid treatingthe dried silicate particles converts the soda to sodium sulfate whichis subsequently removed by washing.

As will be illustrated in the examples, the order'in which sodaneutralization is carried out has a significant influence on theultimate surface area of the final product. For example, if the spraydried particles are first slurried in water prior to acid treatment, ahigh surface area, above about 200 mF/g. will result. On the other hand,if the dried particles are first treated with a neutralizing amount ofacid, a surface area of less than 100 m. /g., and generally of the orderof about 10 to 50 m. /g., will be obtained.

If the sodium silicate is treated with sufiicient acid to neutralize thesoda prior to spray drying, a product with a surface area of about 200m. g. will result.

In this process, the sodium silicate is treated with sufficient acid toneutralize the alkali in the silicate but not suflicient to form a gel.The neutralization is carried out with a solution of a strong mineralacid, such as hydrochloric, sulfuric, nitric, etc. The pH is adjusted toabout 2.9 to 3.5 in this step. After the neutralization, the sol isspray dried and washed using the techniques described previously.

The invention is further illustrated by the following ex-, amples.

Example I A 6.0 B. sodium silicate solution having a 3.25:1 silica tosoda weight ratio and at a temperature of F. was passed as a fine mistthrough a spray nozzle into the top portion of a spray dryer. The dryerinlet temperature was 900 F. and the outlet temperature 300-350 F. Themist, upon contact with the heated atmosphere, solidified intospheroidal silicate particles and dropped into the bottom of the dryingzone. Six pounds of the dried product were collected and acidified with6 gallons of a 1.25% ammonium sulfate solution with vigorous agitation,filtered, then washed with 6 gallons of water, re-washed with 3 gallonsof a .125 ammonium sulfate solution and again washed with 9 gallons ofwater. The

thus washed silica particles were then finally dried and analyzed. Theresults of such analysis are as follows:

T.V. (total volatile matter) overall basis, percent 5.42

Na O, dry basis, percent 0.59 S dry basis, percent 0.16

Particle size (corrected air elutriation):

0-20 microns, percent 65.7 20-40 microns, percent 29.7 40-80 microns,percent 3.6 80+ microns, percent 1.0 Average particle size, microns 16Surface measurements at 1000 F.:

Surface area, mF/g. 40 Pore volume, cmfi/g. 0.13 Pore diameter, A 130Example II This example illustrates the effect of treating the spraydried silicate with water before neutralization of the soda with anacidic material is carried out. A sodium silicate solution was spraydried in the same manner as described in Example 1 except that in thiscase a 21.0" B. sodium silicate solution was used. The dried particleswere divided into three portions, designated A, B and C, and eachportion treated as follows:

Portion A.-6 pounds of dried particles were slowly added to a 1%sulfuric acid solution with vigorous agitation. The acid-treatedparticles were then filtered, washed first with 12 gallons of water,then with 6 gallons of a 2% ammonium sulfate solution and again with 18gallons of water and finally re-dried.

Portion B.-To 6 pounds of dried particles there were slowly added 6gallons of Water with vigorous agitation. 2 pounds of 98% sulfuric acidwere then added to the water-slurried particles, filtered, Washed firstwith 12 gallons of water, then with 6 gallons of 2% ammonium sulfatesolution, then with 18 gallons of water and finally re-dried.

Portion C.-In this case, the acid was slowly added with agitation to thesilica instead of the silica addition to the acid as carried out inPortion A. Washing was carried out in the same order and the types andquantities of washing media were the same as described in Portion A.

The final products resulting from each of the above treatments gave thefinal analyses:

T.V., overall basis, percent- 5. 72 4. 72 8. 06 N220, dry basis,percent. 0.08 1. 12 0.14 S0 dry basis, percent 0.11 0. 68 0. 09 Particlesize (Corrected Air Elut 0-20 microns, percent s 42. 1 38. 6 45. 6 20-40microns, percent. 30. 3 22. 7 29. 4 40-80 microns, percent 24.8 22. 280+ microns, percent 13. 0 2. 8 Surface Measurements at 1,000 F.:

Surface area, mfl/g..- 256 12 Pore volume. cmfi/g- .83 02 Pore diameter,A 130 67 Particle density, gJml 79 2. 20

It is apparent from the above that Portion B treatment gave a producthaving a much higher surface area than the products treated according tothe methods of Portions A and C. It is also significant from theanalyses of the products treated according to A and C that the order ofadding the silicate particles to acid or vice versa had little or noinfluence on the surface area of the final product.

Example 111 This example punctuates the effect on the surface area ofthe final product when the sodium silicate solution is first treatedwith acid to neutralize the soda and then spray drying the resultingneutralized mixture. It differs from the process of the inventioncovered in application Serial No. 832,890 which consists in first spraydrying the silicate solution and thereafter neutralizing the soda in thedried particles.

A 6.7 B. sodium silicate solution was acidified with sulfuric acid toform a sol having a pH of 2.9. The resulting sol was aged for one dayand then spray dried before it gelled. Four pounds of the driedparticles were treated with 2 gallons of 5.5% sulfuric acid, then washedfirst with 4 gallons of water, then with 2 gallons of 2% ammoniumsulfate solution, followed with a final wash with 6 gallons of water andfinally re-dried.

The final product analyzed as follows:

T.V. overall basis, percent 6.13 Na O, dry basis, percent 0.023

S0 dry basis, percent 0.32

Kellogg density, lbs./ft.

Loose 27.1 Freely settled 34.5 Compacted 42.8

Surface measurements at 1000 F Surface area, nL /g. 229 Pore volume,cmfi/g. 0.20 Pore diameter, A. 35

Particle size:

4-10 microns, percent 30 10-20 microns, percent 45 -30 microns, percentIt is noted that by first acidifying the silicate solution to form a soland thereafter spray drying the sol, a final product is obtained havinga relatively high surface area. Moreover, an examination of the finalproduct showed that only 80% of the particles were of spheroidal shapewhile 20% appeared in the form of agglomerates.

Example IV This example emphasizes the effect of agitation during acidtreatment of the spray dried silicate particles and the influence ofsuch action on the surface area of the final product. 6 pounds of thespray dried particles prepared according to Example I were treated with3 gallons of 6% sulfuric acid without agitation. The slurry gelled andhad to be broken up. It was then filtered, washed with 6 gallons ofwater, then with 2% sulfuric acid, rewashed with 9 gallons of water, andfinally rcdried. The product analyzed as follows:

T.V., overall basis, percent 6.03 Na O, dry basis, percent .1.44 S0 drybasis, percent 1.88

Particle size (corrected air elutriation):

0-20 microns, percent 62.8 20-40 microns, percent 26.1 -80 microns,percent 8.9 microns, percent 2.2

Surface measurements at 1000 F.:

Surface area, m. g. 250 Pore volume, cm. g 0.98 Pore diameter, A. 157

These data indicate the absence of agitation during acid treatment ofthe dried particles prior to washing tends to localize the action of theacid by redissolving the dried particles and results in a higher surfacearea of the product.

What is claimed is:

1. A process for preparing a silica gel wherein about 80% of theparticles have a particle size below 80 microns and a surface area ofabout to 250 mF/g. which comprises the steps of:

(a) diluting a sodium silicate solution to about 6 B.,

(b) adjusting the pH of the solution to about 2.9 to

3.5 with a mineral acid selected from the group con sisting ofhydrochloric, nitric and sulfuric acids,

(c) aging the resulting sol,

(d) spray drying said sol in a spray drier operated at .an inlettemperature of about 900 F. and an outlet temperature of about 300 F.,

(e) washing the resulting particles with a solution of sulfuric acid inabout a 6% concentration, followed by a water wash and a wash with asolution of ammonium sulfate in a concentration of about 2%,

(f) drying and recovering the product silica.

2. A process for preparing a silica gel wherein about 80% of theparticles have a particle size below about 80 microns and a surface areaof about 100 to 250 square meters per gram which comprises the steps of:

(a) diluting a sodium silicate solution to a concentration of about 6.7B.,

(b) adjusting the pH of the solution to about 2.9 with a strong mineralacid selected from the group consisting of hydrochloric, nitric andsulfuric acids,

(c) aging the resulting 801 at ambient temperature for about 24 hours,

(d) spray drying said 501 in a spray drier operated at (f) drying andrecovering the product silica.

References Cited by the Examiner UNITED STATES PATENTS Stoewener 23-482McClure et a1 23-482 Connolly et al. 23'182 Hay 23182 Ziese 23-182Alexander 23182 Winyall 23182 20 MAURICE A. BRINDISI, Primary Examiner.

1. A PROCESS FOR PREPARING A SILICA GEL WHEREIN ABOUT 80% OF THEPARTICLES HAVE A PARTICLE SIZE BELOW 80 MICRONS AAND A SURFACE AREA OFABOUT 100 TO 250 M.2/G. WHICH COMPRISES THE STEPS OF: (A) DILUTING ASODIUM SILICATE SOLUTION TO ABOUT 6* BE., (B) ADJUSTING THE PH OF THESOLUTION TO ABOUT 2.9 TO 3.5 WITH A MINERAL ACID SELECTED FROM THE GROUPCONSISTING OF HYDROCHLORIC, NITRIC AND SULFURIC ACIDS, (C) AGING THERESULTING SOL, (D) SPRAY DRYING SAID SOL IN A SPRAY DRIER OPERATED AT ANINLET TEMPERATURE OF ABOUT 900*F. AND AN OUTLET TEMPERATURE OF ABOUT300*F., (E) WASHING THE RESULTING PARTICLES WITH A SOLUTION OF SULFURICACID IN ABOUT AT 6% CONCENTRATION, FOLLOWED BY A WATER WASH AND A WASHWITH A SOLUTION OF AMMONIUM SULFATE IN A CONCENTRATION OF ABOUT 2%, (F)DRYING AND RECOVERING THE PRODUCT SILICA.